1. Field of the Invention
The present invention generally relates to remote units and remote systems, and more particularly, to a remote unit that can display the operation screen of a server located at a distance and a remote system that includes the remote unit.
2. Description of the Related Art
A display device is normally designed to display an input image by scanning an image with three types of image signals R (Red), G (Green), and B (Blue). Along with the image signals, a horizontal synchronization signal (HSYNC) and a vertical synchronization signal (VSYNC) are also inputted. Based on the horizontal synchronization signal (HSYNC) and the vertical synchronization signal (VSYNC), horizontal synchronization and vertical synchronization are established among the image signals.
A display cable, such as an industry standard BNC cable or a D-sub 15-pin cable, therefore includes five signal lines: three signal lines for the three types of image signals, and two signal lines for the two types of synchronization signals.
In a case where synchronization cannot be established among the RGB image signals, however, a color shift or a blur may be caused on the screen of the display device. It is therefore necessary to establish accurate synchronization among the cables for transmitting the image signals.
Japanese Unexamined Utility Model Publication No. 63-133777 (hereinafter referred to as Patent Document 1), for example, discloses a technique of solving shifts of synchronization that are caused by differences in the delay amount among the cables. In accordance with Patent Document 1, digital video signals, a horizontal synchronization digital signal, and a vertical synchronization digital signal, are transmitted through different cables in a digital address display device, and the differences in the delay amount among the cables are corrected.
In recent years, public attention has been drawn to a technique of controlling a server or the like located at a distance of 100 meters or longer from a user. In such a technique, the server located at a distance needs to be somehow connected to the display device, the keyboard, and other devices located near the user. Therefore, a remote unit for transmission and a remote unit for reception are conventionally provided as intermediate nodes between a server and a display device, and a LAN (Local Area Network) cable connects the transmission remote unit and the reception remote unit.
In a system having remote units between a server and a user, data signals to be transmitted and received include the three image signals (RGB), the horizontal synchronization signal, the vertical synchronization signal, and operation data signals inputted through a keyboard and a mouse. However, a standard LAN cable such as a Cat5E cable includes only four signal lines, and cannot transmit all the data signals through the signal lines.
As a method for solving this problem, Japanese Unexamined Patent Publication No. 10-509545 (hereinafter referred to as Patent Document 2) discloses a technique of superimposing an image signal with either one of the synchronization signals prior to transmission.
In a case where an image signal is superimposed on a synchronization signal, however, it is difficult to maintain synchronization among the image signals by the technique disclosed in Patent Document 1. This is because Patent Document 1 concerns a structure for maintaining phase synchronization among signals, but does not concern a structure for establishing phase synchronization among signals contained in superimposed signals.
Also, the technique disclosed in Patent Document 1 is a digital video technique that does not concern a structure such as an analog display device in which the RGB image signals are independent of one another.